The present invention relates to a process for smoothening, in particular for lapping and/or polishing materials comprising III-N surfaces, particularly III-N substrates or III-N templates comprising foreign substrates. Here, N means nitrogen and III means at least one element of the III. main group of the Period System, selected from aluminum, gallium and indium (in the following partially abbreviated by (Al, Ga, In)). The invention further relates to III-N substrates and foreign substrate-comprising III-N templates. These III-N substrates and III-N templates are well suited as substrates or templates for manufacturing electronic devices/components.
For a long time, mechanical and/or chemical-mechanical polishing have been commercially used standard processes for planarizing and smoothening the surfaces of semiconductor substrates, such as of GaAs. To this end, smooth and planar as well as defect-free substrate surfaces are prerequisites for subsequent epitaxy or lithography steps for the manufacture of micro-electronic or opto-electronic devices/components. Known polishing processes for GaN-substrates having c-plane orientation differ due to different chemical stabilities for the Ga-polar ([0001]-orientation) and the N-polar ([000 1]-orientation) surface, respectively. Thus, the Ga-surface (i.e. [0001]) is almost chemically inert at room temperature, whereas the N-surface (i.e. [000 1]) is affected by various etchants (for example aqueous NaOH or KOH solutions). In addition, the Ga-surface is significantly harder than the N-surface.
Weyher et al. (Chemical Polishing of Bulk and Epitaxial GaN”, J. Cryst. Growth 182 (1997) 17) presented a polishing process for polishing the N-surface (remark: a correlation of the results with the N- or the Ga-surface respectively was made later on, see e.g. J. Weyher et al. “Defects in GaN Single Crystals and Homoepitaxial Structures”, J. Cryst. Growth 281 (205, 135), consisting of a step of mechanically polishing by using diamond slurry and a subsequent step of CMP by using an aqueous solution of KOH and/or NaOH. The Ga-surface, however, for which the said process is not applicable, is of unequally higher importance for the subsequent epitaxy (see e.g. Miskys et al., “MOCVD-Epitaxy on Free-Standing HVDE-GaN-Substrates”, Fris. phys. stat. sol. (a) 176 (1999, 443). Weyher et al. did not indicate the hard material used for the abrasive particles.
The process described by Porowsky et al. (“Mechano-Chemical Polishing of Crystals and Epitaxial Layers of GaN and Ga1-x-yAlxInyN”, U.S. Pat. No. 6,399,500) corresponds to the aforementioned prior art of Weyher et al. There is no explicit hint to the polarity of the polished surface. The hard material used for the abrasive particles is not indicated.
Tavernier et al. (“Chemical Mechanical Polishing of Gallium Nitride”, Electrochemical and Solid-State Letters 5 (2002) G61) report about a CMP-process by using silicon oxide as abrasive particles, which process is also successfully applicable only for the N-surface of GaN, but is unsuited for the Ga-surface.
Karouta et al. (“Final Polishing of Ga-Polar GaN-Substrates using Reactive Ion Etching”, Journal of Electronic Materials 28 (1999) 1448) presented a process for polishing the Ga-surface of GaN by means of reactive ion etching (RIE), wherein the crystals have been pre-treated with diamond slurry in a previous mechanical polishing step. The RIE-process further has the disadvantage that it is very cumbersome, and that furthermore the ion irradiation leads to a damaging of the crystal lattice in the surface-near region.
Kim et al. (“Method for Fabrication GaN-Substrate”, U.S. Pat. No. 6,211,089) report on a process for polishing GaN-substrates, consisting of mechanical polishing steps with diamond slurry and boron carbide plates, wherein also, for abolishing polishing damages, a RIE-process with the aforementioned disadvantages as well as an additional terminal annealing step are used.
Xu et al. (“High Surface Quality GaN Wafer and Method of Fabricating Same”, U.S. Pat. No. 6,951,695) describe a method for chemical-mechanical polishing, inter alia of AlxGayInz-terminated (0001)-AlxGayInzN-surfaces by using abrasive silicon oxide or aluminum oxide particles in acid or basic solution. From the description, it is further derivable that the structural damages in the crystal, which are caused by the preferred use of diamond slurry (or, alternatively, silicon carbide, boron carbide or aluminum oxide slurry) in the mechanical polishing which precedes the CMP-step, as well as by the CMP-process, are diminished or minimized by a subsequent wet-chemical etching step, for example using 180° C. hot phosphoric acid, which is technologically laborious.
Kato et al. (“Polishing Composition for Semiconductor Substrate and Method of Manufacturing Semiconductor Substrate Using Same”, JP application 2003-100373) describe a slurry composition merely for polishing GaN, consisting of a mixture of hard (e.g. diamond) and soft (e.g. silicon oxide) abrasive particles. In the described polishing process, the slurry is maintained at a temperature between 10° C. and 80° C. Polishing at high temperature is technologically very laborious.